This present invention relates to intercalated graphite flakes having improved expansion characteristics when exposed to elevated temperatures, and a process for preparing such flakes.
Intercalated graphite flake has many useful applications. One common application is to exfoliate the intercalated graphite particles into vermicular-like structures that are then compressed into sheets conventionally referred to as flexible graphite, for use in the manufacture of products such as gaskets, thermal management materials or fuel cell components. Intercalated graphite flake is also used in a variety of products that take advantage of the expansion characteristics of intercalated graphite flake when exposed to high temperature. Examples of such uses include using the intercalated flake in combination with polymer foams to form seat cushions and furniture upholstery in aircraft and incorporation of the flake in resin-based coatings for wallboards, lumber and other construction materials. Upon exposure to fire, the high temperature will cause the particles of intercalated graphite to exfoliate which minimizes or prevents the formation of toxic gases from the polymer foam and may, of itself, smother a fire.
Since it is important to suppress, i.e. retard, a fire before it has begun to spread, it would be a substantial advantage for an intercalated graphite flake product to exhibit a very high degree of exfoliation upon exposure to temperatures as low as 600xc2x0 C. and even lower. Moreover, since in some applications the graphite flake is incorporated in another material, such as a polymer foam or a resin, the foam or resin can tend to physically inhibit expansion, i.e., there is a load on the flake when it expands. In such circumstances, not only is the degree of expansion important, so is the expansion pressure. Insufficient expansion pressure will result in reduced expansion under the load to which the flake is subjected, reducing the fire-suppression effectiveness of the intercalated graphite flake.
It has been discovered in accordance with the present invention that the treatment of intercalated graphite flake with surfactant, following intercalation of the graphite flake with an oxidizing intercalant solution results in a material which exhibits enhanced exfoliation characteristics, such as expansion pressures, at exfoliation temperatures as low as 600xc2x0 C. and even lower.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Particles, or flakes, of graphite can be treated by a process referred to as intercalation to form intercalated graphite flake whereby, after exposure to elevated temperatures, the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
The present invention provides a method for forming particles of intercalated graphite flake having enhanced exfoliation characteristics, specifically, expansion pressure, at temperatures as low as 600xc2x0 C. and even lower by treating particles of graphite with an oxidizing intercalant solution to provide intercalated graphite flake; washing the intercalated graphite flake; and thereafter contacting the intercalated graphite flake with a surfactant.